This invention relates to optical fibers and optical waveguides, and, more particularly, to precision control of the temperature and/or the optical length of an optical fiber or optical waveguide.
An optical fiber typically includes a core of a first glass, a casing of a second glass overlying the core, and a protective layer overlying the casing. Light introduced into the core is propagated by an internal reflection mechanism along the length of the optical fiber, following the path of the optical fiber with essentially no loss of energy. The light may be propagated over great distances and through complex paths. These same properties may also be obtained using optical waveguides, which are typically etched integrated optics devices formed on a substrate. As used herein, the term "optical fiber" will be understood to encompass both discrete optical fibers and integrated optical waveguides, unless the context indicates the contrary.
In some applications of optical fibers, it is important to determine and control the temperature and/or the optical length of the optical fiber very precisely. (The "optical length" is the product of the physical length of the optical fiber and its index of refraction.) The temperature and optical length of the optical fiber are linked through the effect of temperature on the refractive index of the glass and through the temperature coefficient of expansion of the glass of the optical fiber.
As an example of an application, an interferometer may be made with one or both of the two interfering light paths being optical fibers. The relative optical lengths of the two light paths of the interferometer must be controlled very precisely for many applications. Because the optical length of the optical fiber is a function of the temperature, one approach for controlling the optical length is to heat the optical fiber to increase its optical length and to cool the optical fiber to decrease its optical length. It may also be necessary to dynamically heat or cool the optical fiber to stabilize its optical length against changes in the environment.
Various types of heaters may be used to heat the optical fiber by applying heat, and to cool the optical fiber by reducing the heat input. In one type of heater, a wire is wrapped around the optical fiber. Thin film heaters contacting the optical fiber are also used. A flat-sided fiber may be heated by an applied electric field. These different approaches work to varying degrees, but have disadvantages in respect to response times for temperature changes, difficulty and cost of fabrication and maintenance, insertion loss, and physical length limitations on the optical length of the optical fiber.
There is a need for an improved approach to controlling the temperature and the optical length of an optical fiber. The present invention fulfills this need, and further provides related advantages.